[Candela] 探索Wi-Fi7的新技術-Preamble Puncturing
Candela的工程師Deepika Balla利用LANforge探索Wi-Fi7中的一大新功能-Preamble Puncturing。文章中從封包去分析Preamble Puncturing展現的行為及探討潛在的優勢。
What is Preamble Puncturing?
Preamble puncturing allows access points (APs) to exclude specific subchannels affected by interference, while continuing transmission across the remaining spectrum. This method maximizes bandwidth utilization, ensuring that interference does not unnecessarily degrade performance across the entire channel. It's particularly useful in environments with a lot of interference or congestion, such as dense urban areas or enterprise networks.
Legacy Bandwidth Management:
Before the introduction of preamble puncturing, APs relied on dynamic bandwidth management. In 802.11ac and prior standards, AP can change the channel width for transmission based on each frame.
In the example shown above, the AP is transmitting on channel 36 with an 80 MHz bandwidth. It can listen packets from surrounding APs or STAs on channels 40, 44, and 48. With legacy dynamic bandwidth management, the AP can fall back to transmitting on its primary 20 MHz channel, instead of suspending the entire 80 MHz transmission. Even if there is no interference on channels 44 or 48, the AP transmits only on the primary 20 MHz channel, as traditional channel bonding requires channel continuity. Let’s look at the bandwidth drop in packet capture.
Initially, when download traffic was running between the AP and the client, the transmission occurred over 80 MHz. However, once interference traffic was introduced, the transmission fell back to 20 MHz on the primary channel. Even though channels 40 and 48 had no interference, the AP continued transmitting only on the primary 20 MHz channel, leading to inefficient spectrum utilization.
The above snapshot clearly shows the bandwidth drop when interference traffic was started.
Preamble Puncturing: Pcap Analysis
- Preamble Puncturing information in beacon frame
The AP indicates puncturing on one of the non-primary 20MHz channels through the Disabled Subchannel Bitmap field within the EHT Operation element in the beacon.
- Preamble Puncturing information in QoS Data frame
In the Radiotap Header of the QoS Data frame, we can find the punctured channel information in EHT MU PPDU in the U-SIG field.
Definition of Punctured Channel Information field in QoS Data frame:
Preamble Puncturing: 5GHz Testing
Now with the 802.11be standard, let’s look at preamble puncturing in 5GHz:
AP Configuration is same as in previous case except that the AP is in 802.11be mode supporting preamble puncturing.
Initially, preamble puncturing was disabled on the AP. The transmission took place in 80MHz and when interference traffic started, it fell back to 20MHz.
When the preamble puncturing was enabled, and the interference traffic was started, the AP disabled the 20MHz interference subchannel (channel 44) and the transmission continued to utilize the 80MHz bandwidth.
It's important to note that the bandwidth is not simply reduced from 80 MHz to 60 MHz, which would be inefficient. The preamble is sent across all 20 MHz subchannels, except for the one impacted by interference.
As shown above, the AP sets the 'Disabled Subchannel Bitmap Present' field to 'True'. It announces the disabled subchannel through the 'Disabled Subchannel Bitmap' field, which is set to 0x0004 for channel 44 in our case.
We can see the Punctured Channel Information field in the QoS Data frame, where the value is set to ‘0x03’, indicating that the channel 44 has been disabled.
The overall bandwidth remained at 80 MHz, rather than falling back to 20 MHz. In this case, the entire 80 MHz bandwidth can be utilized for transmission except for the punctured 20 MHz.
Hence preamble puncturing enhances Wi-Fi performance by allowing efficient use of available bandwidth, even in interference-prone environments.
We're currently testing preamble puncturing across different access points and have observed some inconsistent outcomes. We'll be conducting further analysis on its impact on throughput (with preamble puncturing enabled and disabled), as well as puncturing in the 6GHz band with 160/320 MHz bandwidths, and will share our findings soon.
Candela Technologies has recently added the TR-398 Issue 4 Preamble Puncturing test case to our suite of testing solutions. Please refer to the attached report:
TR-398 Issue-4 Preamble Puncturing Test Report.pdf
邁訊科技與Candela致力於對最新的Wi-Fi技術提供成本可控的測試解決方案。LANforge協助產品製造商在研發及驗證階段做長時間大量且複雜的自動化測試，包含Rate vs. Range (RvR)，Rate vs. Range vs. Orientation (RvRvO)，Capacity (Multi-client)，dual-band，Mesh，RFC2544等。在認證方面Candela也是Broad forum的會員，針對TR398i1, TR398i2, TR398i3, TR398i4提供自動化的完整測試方案。
邁訊科技Meshlink自2023年WiFi7測試方案發表後已經與Candela攜手協助國內外各大網通設備製造商進行Wi-Fi7的測試及驗證。Wi-Fi7帶來新的技術及測試挑戰，MLO（多重連結作業）、MRU、4096QAM調變、320MHz頻寬，新的調變技術、更高的頻寬帶來更高的吞吐量Throughput。Meshlink邁訊科技與Candela Technology將協助客戶一同面對新的挑戰，並於近期佈建WiFi7 demo環境供客戶一起探索最新的技術。請隨時關切邁訊科技網站資訊或隨時與我們的技術窗口聯繫(support@meshlink.com.tw)。
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